MRAM is a non-volatile memory technology that stores data through magnetic storage elements. Because MRAM is non-volatile, memory written thereto may be retained even when a power supply of the MRAM is turned off. The magnetic storage elements used to actually store the data include two ferromagnetic plates, or electrodes, that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates is referred to as the reference layer and has a magnetization which is pinned. In other words, the reference layer has a higher coercivity than the other plate and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and has a magnetization direction which can be changed by relatively smaller magnetic fields or a spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a logical “1” or a logical “0” can be stored in each respective MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of a cell changes due to the orientation of the magnetic fields of the two layers. The resistance of a cell will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a logical “1” and a logical “0”.
An important and continuing goal in the data storage industry is that of increasing the density of data stored on a medium. For storage devices which implement MRAM, that goal has led to decreasing the footprint of individual MRAM cells in an attempt to further increase the storage capacity per unit area. However, when forming increasingly smaller memory elements, process variability and fabrication imperfections may cause the characteristics of the resulting structure to have an effective resistance, thickness, surface roughness, etch profile, etc. that is different than intended. It follows that quality control for these memory element structures has an effect on the performance of the formed structure.
Furthermore, variations resulting during fabrication may have a relatively localized effect in some instances, while in others the variations may affect the resulting structure on more of a global scale depending on the cause of the variation which occurred. One such causes of variation includes point defects and/or clusters during layer deposition which will affect local thicknesses of the deposited layer around the point defects. Another cause of variation includes variation in the deposition condition of the chamber, e.g., including gas flow rates, pressure control, deposition temperature, RF plasma, etc.